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Universe, Volume 5, Issue 9 (September 2019)

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Cover Story (view full-size image) Hawking’s prediction that black holes radiate indicates a basic inconsistency with quantum laws. [...] Read more.
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Open AccessArticle
An Overview of Quasinormal Modes in Modified and Extended Gravity
Universe 2019, 5(9), 202; https://doi.org/10.3390/universe5090202 - 19 Sep 2019
Viewed by 245
Abstract
As gravitational waves are now being nearly routinely measured with interferometers, the question of using them to probe new physics becomes increasingly legitimate. In this article, we rely on a well established framework to investigate how the complex frequencies of quasinormal modes are [...] Read more.
As gravitational waves are now being nearly routinely measured with interferometers, the question of using them to probe new physics becomes increasingly legitimate. In this article, we rely on a well established framework to investigate how the complex frequencies of quasinormal modes are affected by different models. The tendencies are explicitly shown for both the pulsation and the damping rate. The goal is, at this stage, purely qualitative. This opportunity is also taken to derive the Regge-Wheeler equation for general static and spherically symmetric metrics. Full article
(This article belongs to the Special Issue Probing New Physics with Black Holes)
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Open AccessFeature PaperArticle
Searching for Quantum Black Hole Structure with the Event Horizon Telescope
Universe 2019, 5(9), 201; https://doi.org/10.3390/universe5090201 - 17 Sep 2019
Viewed by 93
Abstract
The impressive images from the Event Horizon Telescope (EHT) sharpen the conflict between our observations of gravitational phenomena and the principles of quantum mechanics. Two related scenarios for reconciling quantum mechanics with the existence of black hole-like objects, with “minimal” departure from general [...] Read more.
The impressive images from the Event Horizon Telescope (EHT) sharpen the conflict between our observations of gravitational phenomena and the principles of quantum mechanics. Two related scenarios for reconciling quantum mechanics with the existence of black hole-like objects, with “minimal” departure from general relativity and local quantum field theory, have been explored; one of these could produce signatures visible to EHT observations. A specific target is temporal variability of images, with a characteristic time scale determined by the classical black hole radius. The absence of evidence for such variability in the initial observational span of seven days is not expected to strongly constrain such variability. Theoretical and observational next steps towards investigating such scenarios are outlined. Full article
(This article belongs to the Special Issue Probing New Physics with Black Holes)
Open AccessArticle
Gauss–Bonnet Inflation and the String Swampland
Universe 2019, 5(9), 200; https://doi.org/10.3390/universe5090200 - 15 Sep 2019
Viewed by 154
Abstract
The swampland criteria are generically in tension with single-field slow-roll inflation because the first swampland criterion requires small tensor-to-scalar ratio while the second swampland criterion requires either large tensor-to-scalar ratio or large scalar spectral tilt. The challenge to single-field slow-roll inflation imposed by [...] Read more.
The swampland criteria are generically in tension with single-field slow-roll inflation because the first swampland criterion requires small tensor-to-scalar ratio while the second swampland criterion requires either large tensor-to-scalar ratio or large scalar spectral tilt. The challenge to single-field slow-roll inflation imposed by the swampland criteria can be avoided by modifying the relationship between the tensor-to-scalar ratio and the slow-roll parameter. We show that the Gauss–Bonnet inflation with the coupling function inversely proportional to the potential overcomes the challenge by adding a constant factor in the relationship between the tensor-to-scalar ratio and the slow-roll parameter. For the Gauss–Bonnet inflation, while the swampland criteria are satisfied, the slow-roll conditions are also fulfilled, so the scalar spectral tilt and the tensor-to-scalar ratio are consistent with the observations. We use the potentials for chaotic inflation and the E-model as examples to show that the models pass all the constraints. The Gauss–Bonnet coupling seems a way out of the swampland issue for single-field inflationary models. Full article
(This article belongs to the Special Issue Inflation, Black Holes and Gravitational Waves)
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Open AccessArticle
Mapping the Narrow-Line Seyfert 1 Galaxy 1H 0323+342
Universe 2019, 5(9), 199; https://doi.org/10.3390/universe5090199 - 14 Sep 2019
Viewed by 138
Abstract
Taking advantage of the most recent measurements by means of high-resolution radio observations and other multiwavelength campaigns, it is possible to elaborate a detailed map of the narrow-line Seyfert 1 Galaxy 1H 0323+342. This map will open the possibility of [...] Read more.
Taking advantage of the most recent measurements by means of high-resolution radio observations and other multiwavelength campaigns, it is possible to elaborate a detailed map of the narrow-line Seyfert 1 Galaxy 1H 0323 + 342 . This map will open the possibility of intriguing hypotheses about the generation of high-energy γ rays in the narrow-line region. Full article
(This article belongs to the Special Issue Seyfert Galaxies Astrophysics)
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Open AccessArticle
Static State of a Black Hole Supported by Dark Matter
Universe 2019, 5(9), 198; https://doi.org/10.3390/universe5090198 - 13 Sep 2019
Viewed by 158
Abstract
The possibility of an equilibrium state of a gravitating scalar field (describing ordinary matter) inside a black hole, compressed to the state of boson condensate, in balance with a longitudinal vector field (describing dark matter) from the outside, is considered. Analytical analysis, confirmed [...] Read more.
The possibility of an equilibrium state of a gravitating scalar field (describing ordinary matter) inside a black hole, compressed to the state of boson condensate, in balance with a longitudinal vector field (describing dark matter) from the outside, is considered. Analytical analysis, confirmed numerically, shows that there are regular static solutions to the Einstein equations with no limitation on the mass of a black hole. The metric tensor component grr(r) changes sign twice. The behavior of the gravitational field and material fields in the vicinity of these two Schwarzschild radii were studied in detail. The equality of the energy–momentum tensors of the scalar and longitudinal vector fields at the interface supports the phase equilibrium of a black hole and dark matter. Considering the gravitating scalar field as an example, a possible internal structure of a black hole and its influence on the dark matter at the periphery of a galaxy are clarified. In particular, the speed on the plateau of a galaxy rotation curve as a function of a black hole’s mass is determined. Full article
(This article belongs to the Special Issue Black Hole Physics and Astrophysics)
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Open AccessArticle
Hot Accretion Flow in Two-Dimensional Spherical Coordinates: Considering Pressure Anisotropy and Magnetic Field
Universe 2019, 5(9), 197; https://doi.org/10.3390/universe5090197 - 12 Sep 2019
Viewed by 113
Abstract
For systems with extremely low accretion rate, such as Galactic Center Sgr A* and M87 galaxy, the ion collisional mean free path can be considerably larger than its Larmor radius. In this case, the gas pressure is anisotropic to magnetic field lines. In [...] Read more.
For systems with extremely low accretion rate, such as Galactic Center Sgr A* and M87 galaxy, the ion collisional mean free path can be considerably larger than its Larmor radius. In this case, the gas pressure is anisotropic to magnetic field lines. In this paper, we pay attention to how the properties of outflow change with the strength of anisotropic pressure and the magnetic field. We use an anisotropic viscosity to model the anisotropic pressure. We solve the two-dimensional magnetohydrodynamic (MHD) equations in spherical coordinates and assume that the accretion flow is radially self-similar. We find that the work done by anisotropic pressure can heat the accretion flow. The gas temperature is heightened when anisotropic stress is included. The outflow velocity increases with the enhancement of strength of the anisotropic force. The Bernoulli parameter does not change much when anisotropic pressure is involved. However, we find that the energy flux of outflow can be increased by a factor of 20 in the presence of anisotropic stress. We find strong wind (the mass outflow is about 70% of the mass inflow rate) is formed when a relatively strong magnetic field is present. Outflows from an active galactic nucleus can interact with gas in its host galaxies. Our result predicts that outflow feedback effects can be enhanced significantly when anisotropic pressure and a relatively powerful magnetic field is considered. Full article
(This article belongs to the Special Issue Black Hole Physics and Astrophysics)
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Open AccessCommunication
Open-Charm Hadron Measurements in Au+Au Collisions at √sNN = 200 GeV by the STAR Experiment
Universe 2019, 5(9), 196; https://doi.org/10.3390/universe5090196 (registering DOI) - 07 Sep 2019
Viewed by 182
Abstract
Study of the open-charm hadron production in heavy-ion collisions is crucial for understanding the properties of the Quark-Gluon Plasma. In these papers, we report on a selection of recent STAR measurements of open-charm hadrons in Au+Au collisions at sNN=200GeV [...] Read more.
Study of the open-charm hadron production in heavy-ion collisions is crucial for understanding the properties of the Quark-Gluon Plasma. In these papers, we report on a selection of recent STAR measurements of open-charm hadrons in Au+Au collisions at s NN = 200 GeV , using the Heavy-Flavor Tracker. In particular, the nuclear modification factors of D 0 and D ± mesons, elliptic and directed flow of D 0 mesons, D s /D 0 and Λ c / D 0 yield ratios are discussed. The observed suppression of D 0 and D ± mesons suggests strong interactions of the charm quarks with the QGP. The measured elliptic flow of D 0 mesons is large and follows the NCQ scaling, suggesting that charm quarks may be close to thermal equilibrium with the QGP medium. Both D s /D 0 and Λ c / D 0 yield ratios are found to be enhanced in Au+Au collisions. The enhancement can be explained by models incorporating coalescence hadronization of charm quarks. In addition, the directed flow of the D 0 mesons is measured to be negative and larger than that of light-flavor mesons which is in a qualitative agreement with hydrodynamic model predictions with a tilted QGP bulk. Full article
(This article belongs to the Special Issue The Zimányi School and Analytic Hydrodynamics in High Energy Physics)
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Open AccessArticle
Nonlocal Gravitomagnetism
Universe 2019, 5(9), 195; https://doi.org/10.3390/universe5090195 - 05 Sep 2019
Viewed by 127
Abstract
We briefly review the current status of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. Nonlocal gravity thus involves integro-differential field equations and a causal constitutive [...] Read more.
We briefly review the current status of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. Nonlocal gravity thus involves integro-differential field equations and a causal constitutive kernel that should ultimately be determined from observational data. We consider the stationary gravitational field of an isolated rotating astronomical source in the linear approximation of nonlocal gravity. In this weak-field and slow-motion approximation of NLG, we describe the gravitomagnetic field associated with the rotating source and compare our results with gravitoelectromagnetism (GEM) of the standard general relativity theory. Moreover, we briefly study the energy-momentum content of the GEM field in nonlocal gravity. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
Open AccessArticle
A Particle Emitting Source From an Accelerating, Perturbative Solution of Relativistic Hydrodynamics
Universe 2019, 5(9), 194; https://doi.org/10.3390/universe5090194 - 04 Sep 2019
Viewed by 158
Abstract
The quark gluon plasma is formed in heavy-ion collisions, and it can be described by solutions of relativistic hydrodynamics. In this paper we utilize perturbative hydrodynamics, where we study first order perturbations on top of a known solution. We investigate the perturbations on [...] Read more.
The quark gluon plasma is formed in heavy-ion collisions, and it can be described by solutions of relativistic hydrodynamics. In this paper we utilize perturbative hydrodynamics, where we study first order perturbations on top of a known solution. We investigate the perturbations on top of the Hubble flow. From this perturbative solution we can give the form of the particle emitting source and calculate observables of heavy-ion collisions. We describe the source function and the single-particle momentum spectra for a spherically symmetric solution. Full article
(This article belongs to the Special Issue The Zimányi School and Analytic Hydrodynamics in High Energy Physics)
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Open AccessArticle
Identifying Quark Matter in Hybrid Stars through Relativistic Tidal Deformations
Universe 2019, 5(9), 193; https://doi.org/10.3390/universe5090193 - 30 Aug 2019
Viewed by 271
Abstract
We study a specific model of neutron star matter that supports a phase transition to quark matter at high density and examine parameter ranges for consistency with the mass-weighted tidal deformability of Λ˜=300230+420 for a mass [...] Read more.
We study a specific model of neutron star matter that supports a phase transition to quark matter at high density and examine parameter ranges for consistency with the mass-weighted tidal deformability of Λ ˜ = 300 230 + 420 for a mass ratio of q [ 0.73 , 1.0 ] , as inferred from observations of gravitational waves from the binary neutron star merger event GW170817. By using this observation to restrict the parameter space for the equation of state (EoS) model used throughout this study, we aim to assess the possibility of a potential solution to the masquerade and flavor camouflage problems for hybrid EoS models. Assuming the two stars have the same EoS, in which the Dirac-Brueckner-Hartree Fock (DBHF) nuclear model transitions to the vBag quark model, we see if the parameter space of these hybrid model stars are restricted due to the adherence to the reported Λ 1.4 70 , 580 and M m a x [ 2.01 , 2.16 ] M constraints. Upon completion, we find that, while the parameter space for our model does get restricted, it does not ultimately resolve the masquerade and flavor camouflage problems. Full article
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Open AccessReview
Selected Topics in Numerical Methods for Cosmology
Universe 2019, 5(9), 192; https://doi.org/10.3390/universe5090192 - 29 Aug 2019
Viewed by 208
Abstract
The large amount of cosmological data already available (and in the near future) makes the development of efficient numerical codes necessary. Many software products have been implemented to perform cosmological analyses considering one or few probes. The need of multi-task software is rapidly [...] Read more.
The large amount of cosmological data already available (and in the near future) makes the development of efficient numerical codes necessary. Many software products have been implemented to perform cosmological analyses considering one or few probes. The need of multi-task software is rapidly increasing, in order to combine numerous cosmological probes along with their specificity (e.g., astrophysical descriptions and systematic errors). In this work, we mention some of these libraries, bringing out some challenges they will face in the few-percent error era (on the cosmological parameters). We review some concepts of the standard cosmological model, and examine some specific topics on their implementation, bringing, for example, the discussion on how some quantities are numerically defined in different codes. We also consider implementation differences between public codes, mentioning their advantages/disadvantages. Full article
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Open AccessArticle
Kerr Black Holes within a Modified Theory of Gravity
Universe 2019, 5(9), 191; https://doi.org/10.3390/universe5090191 - 28 Aug 2019
Viewed by 234
Abstract
The Kerr black hole is studied within a modified theory of gravity, which adds the effects of vacuum fluctuations near a black hole. These vacuum fluctuations are treated as a dark energy. A parameter is introduced to account for these fluctuations. It is [...] Read more.
The Kerr black hole is studied within a modified theory of gravity, which adds the effects of vacuum fluctuations near a black hole. These vacuum fluctuations are treated as a dark energy. A parameter is introduced to account for these fluctuations. It is zero for the standard theory and acquires a maximal value, just before there would be no event horizon. The existence of an event horizon not only depends on the value of this parameter, but also on the spin of the black hole. In addition, we study the existence of a light-ring. We also elaborate on the relation of the appearance and vanishing of the event horizon and light-ring to phase transitions. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
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Open AccessArticle
Rotation Sensing Lasers in General Relativity: Some Technical Notes and Current Advances
Universe 2019, 5(9), 190; https://doi.org/10.3390/universe5090190 - 21 Aug 2019
Viewed by 276
Abstract
We review the current status of large ring laser gyroscopes having the potential to contribute to terrestrial measurements of general relativistic precessions. At this point in time, although these devices possess the raw sensitivity for such a measurement, they remain limited by long-term [...] Read more.
We review the current status of large ring laser gyroscopes having the potential to contribute to terrestrial measurements of general relativistic precessions. At this point in time, although these devices possess the raw sensitivity for such a measurement, they remain limited by long-term geometric instability, detection noise and imperfections in the physical models required to isolate geophysical effects. Furthermore, minute non-reciprocal biases provide a null-shift error and therefore no currently constructed laser system meets the requirement of absolute rotation rate sensing. Nevertheless, we are of the view that these are surmountable problems and the ability of ring laser gyroscopes to measure low frequency to DC signals has vastly increased in the last decade. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
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